The present invention relates to a laser driving method and a laser driving device, and particularly to a technique for controlling laser power in a recording and reproduction apparatus using a laser.
Recording and reproducing apparatus using a laser as a light source are utilized in various fields. For example, optical disk recording and reproducing apparatus (hereinafter also referred to simply as optical disk apparatus) using an optical disk as a medium for recording and reproduction, optical communication devices, laser printers and the like are drawing attention.
As to the laser used as a light source, semiconductor lasers using a semiconductor element have recently been widely used as a light source for various devices because semiconductor lasers are very small in size and respond quickly to driving current.
Phase change optical disks, magneto-optical disks and the like are widely known as rewritable optical disks used as media for recording and reproduction. Outputs of laser light applied in recording, reproduction, and erasure differ from each other. For example, at the time of recording, laser beam output is increased (for example 30 mW or more) to create recorded marks referred to as pits on an optical disk, whereas at the time of reproduction, the optical disk is irradiated with a weaker laser beam output (for example 3 mW) than at the time of recording to read information without destroying the recorded pits.
In order to obtain an error rate allowing recording and reproduction on a recent optical disk of high density and high transfer rate, it is necessary to properly control these laser beam intensities.
However, the semiconductor laser shows a significant temperature characteristic change in driving current-optical output characteristics of the semiconductor laser. In order to set the optical output of the semiconductor laser to desired intensity, a circuit for controlling the optical output of the semiconductor laser so as to make the optical output of the semiconductor laser constant, or a so-called APC (Auto Power Control) circuit is required.
APC circuits are roughly classified into a real-time system and a sample hold system. In the real-time system, the optical output of the semiconductor laser is monitored by a light receiving element, and an optical and electric negative feedback loop is formed to control a current for driving the semiconductor laser at all times so as to make a received light current (proportional to the optical output of the semiconductor laser) occurring in the light receiving element equal to a light emission command signal.
In the sample hold system, the optical output of the semiconductor laser is monitored by a light receiving element in a power setting period, and an optical and electric negative feedback loop is formed to control a driving current for driving the semiconductor laser so as to make a received light current (proportional to the optical output of the semiconductor laser) occurring in the light receiving element equal to a light emission command signal. A control value for the driving current is retained in other than the power setting period, and modulation is performed on the basis of the retained control value in other than the power setting period.
FIG. 13 is a block diagram showing an example of fundamental configuration of an APC circuit employing the real-time system. An APC circuit 800 is roughly divided into an optical system device 801 and an electric system device 802. A semiconductor laser (LD; Laser Diode) 812 in the optical system device 801 emits laser light L1. Laser light L1a divided via a beam splitter 814 is applied to the surface of an optical disk via a collimator lens 818 and an objective lens 819, whereby recording, reproduction, and erasure are performed.
Also, laser light L1b obtained by dividing the laser light L1 emitted from the semiconductor laser (LD; Laser Diode) 812 by the beam splitter 814 is applied to a photodiode 816 as a light receiving element. A current (IFPD) obtained by photoelectric conversion by the photodiode 816 is transmitted as a signal of a feedback system to the electric system device 802.
A current-to-voltage converting unit (I/V converter) 826 in the electric system device 802 converts the current output (feedback current IFPD) of the photodiode 816 into a voltage signal. In the meantime, a current-to-voltage converting unit (I/V converter) 836 similarly converts a power reference current IREF generated by a reference current generating unit 832 and set by a digital/analog (D/A) converting unit 834 into a voltage signal. Both the voltage signals are input to an error amplifier 840.
In the APC circuit 800 of such a configuration, an APC negative feedback loop as a whole is formed so that the differential inputs to the error amplifier 840 balance each other, and the output of the error amplifier 840 is set to an arbitrary voltage. A voltage-to-current converting unit (V/I converter) 880 converts the voltage into a current signal. A current driving unit (Driver) 892 amplifies the current signal, and then supplies a driving current ILD to the semiconductor laser 812 to drive the semiconductor laser 812.
Thus, in the APC circuit 800, the negative feedback control loop is formed so as to make the feedback current IFPD=the power reference current IREF, the laser driving current ILD is determined by a ratio of an amount ΔIFPD of optical system feedback to ΔILD, and laser light emission power is determined by current-to-light conversion efficiency of the semiconductor laser 812.
It is generally known that a laser oscillates with a driving current higher than a light emission threshold current and that current-to-light conversion sensitivity of a laser shows a substantially constant characteristic.
For example, FIG. 14 is a diagram showing a relation between driving current and laser power. As shown in the figure, the driving current higher than a light emission threshold current Ith causes oscillation, and current-to-light conversion sensitivity ΔPLD/ΔILD shows a substantially constant characteristic. A change in the temperature causes the threshold current to drift greatly with a positive temperature coefficient. The driving current Ild for obtaining desired laser power Pld is expressed by Equation (1). When there is a temperature drift of the light emission threshold current Ith and the current-to-light conversion sensitivity ΔPLD/ΔILD of a laser, the driving current Ild is correspondingly changed.
[Equation 1]Ild=ΔILD/ΔPLD*Pld+Ith  (1)
Hence, as an APC system, a real-time APC system is desirable in which laser power control is performed at all times in each of a recording mode and a reproduction mode.
Mark edge recording, which uses changes at both ends of a mark for recording, has become mainstream because of superiority thereof for achieving higher density of a recent writable optical disk. As a technique for preventing data errors due to mark shape distortion in mark edge recording, a write strategy technique in which multi-pulse modulation of laser output power is performed according to disk material and recording speed is known (see for example Patent Document 1).
[Patent Document 1]
Japanese Patent Laid-Open No. 2000-244054
As a mechanism for performing the multi-pulse modulation of the laser output power when the write strategy technique is employed to drive the laser, there is for example a system (hereinafter referred to also as a superimposition driving system) in which a current pulse component for generating a light emission waveform of multilevel power is superimposed on a bias current component providing the threshold current Ith of the laser, a combined current is thereby generated, and the combined current is supplied to the laser to thereby perform pulse driving of the laser.